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Chapter 7 Fires Within: Igneous Activity The Nature of Volcanic Eruptions Characteristics of a magma determine the “violence” or explosiveness of an eruption Composition Temperature Dissolved gases The above three factors actually control the viscosity of a magma

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the nature of volcanic eruptions
The Nature of Volcanic Eruptions
  • Characteristics of a magma determine the “violence” or explosiveness of an eruption
      • Composition
      • Temperature
      • Dissolved gases
  • The above three factors actually control the viscosity of a magma
the nature of volcanic eruptions3
The Nature of Volcanic Eruptions
  • Viscosity is a measure of a material’s resistance to flow
  • Factors affecting viscosity
      • Temperature—Hotter magmas are less viscous
      • Composition—Silica (SiO2) content
        • Higher silica content = higher viscosity
        • Lower silica content = lower viscosity
the nature of volcanic eruptions4
The Nature of Volcanic Eruptions
    • Dissolved gases
      • Gases expand within a magma as it nears Earth’s surface due to decreasing pressure
      • The violence of an eruption is related to how easily gases escape
  • In summary
    • Basaltic lavas = mild eruptions
    • Rhyolitic or andesitic lavas = explosive eruptions
materials extruded from a volcano
Materials Extruded from a Volcano
  • Lava flows
      • Basaltic lavas exhibit fluidbehavior
      • Types of basaltic flows
        • Pahoehoe lava (resembles a twisted or ropey texture)
        • Aa lava (rough, jagged blocky texture)
  • Dissolved gases
      • 1%–6% by weight
      • Mainly H2O and CO2
aa lava flow
Aa Lava Flow

Figure 7.5 A

materials extruded from a volcano7
Materials Extruded from a Volcano
  • Pyroclastic materials—“Fire fragments”
      • Types of pyroclastic debris
        • Ash and dust—Fine, glassy fragments
        • Pumice—Porous rock from “frothy” lava
        • Cinders—Pea-sized material
        • Lapilli—Walnut-sized material
        • Particles larger than lapilli
          • Blocks—Hardened or cooled lava
          • Bombs—Ejected as hot lava
a volcanic bomb
A Volcanic Bomb

Figure 7.6 (top)

volcanic structures
Volcanic Structures
  • General features
      • Opening at the summit of a volcano
        • Crater—Summit depression <1 km diameter
        • Caldera—Summit depression >1 km diameter produced by collapse following a massive eruption
      • Vent—Surface opening connected to the magma chamber
      • Fumarole—Emit only gases and smoke
volcanic structures10
Volcanic Structures
  • Types of volcanoes
      • Shield volcano
        • Broad, slightly domed shape
        • Generally cover large areas
        • Produced by mild eruptions of large volumes of basaltic lava
        • Example = Mauna Loa on Hawaii
volcanic structures12
Volcanic Structures
  • Cinder cone
    • Built from ejected lava (mainly cinder-sized) fragments
    • Steep slope angle
    • Small size
    • Frequently occur in groups
volcanic structures14
Volcanic Structures
  • Composite cone (stratovolcano)
    • Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mount St. Helens)
    • Large, classic-shaped volcano (1000s of ft. high and several miles wide at base)
    • Composed of interbedded lava flows and pyroclastic debris
    • Most violent type of activity (e.g., Mt. Vesuvius)
volcanic structures18
Volcanic Structures
  • Nuée ardente
    • Nuée ardente—A deadly pyroclastic flow
        • Fiery pyroclastic flow made of hot gases infused with ash and other debris
        • Also known as glowing avalanches
        • Move down the slopes of a volcano at speeds up to 200 km per hour
volcanic structures20
Volcanic Structures
  • Lahar—Volcanic mudflow
      • Mixture of volcanic debris and water
      • Move down stream valleys and volcanic slopes, often with destructive results
other volcanic landforms
Other Volcanic Landforms
  • Caldera
      • Steep-walled depressions at the summit
      • Generally >1 km in diameter
      • Produced by collapse
  • Pyroclastic flow
      • Felsic and intermediate magmas
      • Consists of ash, pumice, and other debris
      • Example = Yellowstone Plateau
other volcanic landforms23
Other Volcanic Landforms
  • Fissure eruptionsandlava plateaus
      • Fluid basaltic lava extruded from crustal fractures called fissures
      • Example = Columbia River Plateau
  • Lava domes
      • Bulbous mass of congealed lava
      • Associated with explosive eruptions of gas-rich magma
other volcanic landforms24
Other Volcanic Landforms
  • Volcanic pipes and necks
      • Pipes—Short conduits that connect a magma chamber to the surface
      • Volcanic necks (e.g., Shiprock, New Mexico)—Resistant vents left standing after erosion has removed the volcanic cone
intrusive igneous activity
Intrusive Igneous Activity
  • Most magma is emplaced at depth in the Earth
      • Once cooled and solidified, is called a pluton
  • Nature of plutons
      • Shape—Tabular (sheetlike) vs. massive
      • Orientation with respect to the host (surrounding) rock
        • Concordant vs. discordant
intrusive igneous activity26
Intrusive Igneous Activity
  • Types of intrusive igneous features
      • Dike—A tabular, discordant pluton
      • Sill—A tabular, concordant pluton (e.g., Palisades Sill in New York)
      • Laccolith
        • Similar to a sill
        • Lens or mushroom-shaped mass
        • Arches overlying strata upward
igneous structures
Igneous Structures

Figure 7.22 B

intrusive igneous activity29
Intrusive Igneous Activity
  • Intrusive igneous features continued
      • Batholith
        • Largest intrusive body
        • Surface exposure >100+ km2 (smaller bodies are termed stocks)
        • Frequently form the cores of mountains
plate tectonics and igneous activity
Plate Tectonics and Igneous Activity
  • Global distribution of igneous activity is not random
      • Most volcanoes are located within or near ocean basins
      • Basaltic rocks = oceanic and continental settings
      • Granitic rocks = continental settings
plate tectonics and igneous activity32
Plate Tectonics and Igneous Activity
  • Igneous activity at plate margins
      • Spreading centers
        • Greatest volume of volcanic rock is produced along the oceanic ridge system
        • Mechanism of spreading
          • Decompression melting occurs as the lithosphere is pulled apart
          • Large quantities of basaltic magma are produced
plate tectonics and igneous activity33
Plate Tectonics and Igneous Activity
  • Subduction zones
    • Occur in conjunction with deep oceanic trenches
      • An island arc if in the ocean
      • A volcanic arc if on a continental margin
    • Associated with the Pacific Ocean Basin
      • Region around the margin is known as the “Ring of Fire”
      • Majority of world’s explosive volcanoes
plate tectonics and igneous activity34
Plate Tectonics and Igneous Activity
  • Intraplate volcanism
      • Occurs within a tectonic plate
      • Localized volcanic region in the overriding plate is called a hot spot
        • Produces basaltic magma sources in oceanic crust (e.g., Hawaii and Iceland)
        • Produces granitic magma sources in continental crust (e.g., Yellowstone Park)